This invention relates to the method of safely storing the compressed gas of a dual bottle subsea accumulator within the gas bottle when the hydraulic bottle must be removed for servicing.
The field of this invention is providing accumulated power for operating blowout preventers in deep water operations to seal the well bore and protect the environment in emergency situations when an obstruction is in the well bore.
Many accumulators are of a single bottle type which separates compressed gas from the hydraulic working fluid by a single boundary. The boundary can be a rubber bladder much like a balloon or a floating steel piston. The advantage of this design is simplicity in that the pressure with the hydraulic fluid is exactly the same as the pressure within the compressed gas. There are various disadvantages of this solution in the subsea environment such as requiring high gas pressures and the need to adjust the gas pressure for surface testing and actual subsea operations.
Depth compensated accumulators such as are described in U.S. Pat. No. 6,202,753 FIG. 3 provide the advantages of lower gas pressures and automatic adjustment for the environmental pressures, however, are more complex in design. As shown in U.S. Pat. No. 6,202,753 FIG. 3, the accumulator is of a single bottle design. However, in practice, the upper gas portion of the bottle is more economically manufactured as a separate gas bottle and the lower piston areas manufactured in a hydraulic bottle—a dual bottle accumulator.
An advantage of the dual bottle accumulator is that it makes the critical tolerance and active part of the accumulator smaller and easier to service. When this servicing is to be done, all the compressed gas within the gas bottle must be vented to the atmosphere and lost. After servicing, fresh nitrogen must be supplied and recompressed up to the storage pressures, i.e. 4500 p.s.i. In an offshore installation, the amount of gas required for pressurizing these large accumulators requires that a large expensive nitrogen manufacturing plant be installed on the rig.